1. Field of the Invention
The present invention relates to a precision device for soft clipping AC and DC signals. Such a device could be used, for example, in an audio power amplifier to limit large amplitudes, while generating less harmonic distortion than would occur with uncontrolled clipping. 2. Description of the Related Art
Soft clipping permits the linear amplification of an audio signal up to a particular amplitude. Thereafter, instantaneous, automatic compression of larger signals is progressively performed. The term "soft clipping" thus refers to the fact that the output signal is not strictly limited to a maximum value as in conventional clipping circuits. Rather, once the output signal reches a threshold value, amplification of the input signal continues to occur, but with a reduced gain.
Several benefits result from the use of soft cliping. Among these are the instantaaneous recovery from peak inputs without the profuse harmonics associated with uncontrolled clipping, and an increase in the maximum input voltage before complete saturation occurs. Further features and advantages of the soft clipping approach for audio signals are discussed in the 1970 IEEE article "`Soft Limiting`in Audio Power Amplification" by Gerald E. Wentworth.
Typical prior art devices for effecting soft clipping have used the non-linearity of forward biased diodes to produce a transfer function that aapproximates a desired curve. For soft clipping, the transfer function is generally represented by a curve of decreasing slope. This approach has inherent limitations, however. For example, the amplitude at which clipping occurs is restricted since the diode voltages are limited. In addition, the accuracy of the transfer curve associated with the soft clipping device is dependent on the diode transfer curve.
British Pat. Publication No. 1,376,961 discloses a DC function generator for producing an input-output characteristic which is set to approximate a desired curve. A plurality of diode equivalent circuits are connected in parallel between the input of an operational amplifier and a summing circuit. Each such circuit is biased to conduct with a particular resistance at a particular input voltage amplitude. With this arrangement, a piecewise linear approximation of a desired transfer curve can be obtained. Such a device entails the use of diode equivalent circuits, ech of which comprises an additional operational amplifier. The complexity of such circuitry renders it inefficient when the input signals are of AC type, such as audio signals, rather than the DC input signals for which it is designed. In addition, a precision voltage source is required for the diode equivalent circuit. The use of plural diodes in the operational amplifier feedback path is also plagued with associated inaccuracies. Although additional diodes can be introduced into the circuit to compensate for voltage drops across the primary diodes, these additional diodes can not be assumed to have matching characteristics which would eliminate all errors.